Final answer:
Repeated stimulation of a muscle preventing complete relaxation between contractions results in incomplete tetanus, leading to complete tetanus if the stimulation frequency is high enough to eliminate the relaxation phase entirely.
Step-by-step explanation:
When a muscle is stimulated repeatedly so that it cannot completely relax between contractions, incomplete tetanus occurs. If the stimulus frequency increases further, eliminating the relaxation phase entirely, the muscle enters a state of complete tetanus.
The process of incomplete or complete tetanus is caused by an increase in the frequency of motor neuron signaling, which leads to a rise in muscle tension in the motor unit. As the tension increases, it may reach a peak about three to four times greater than a single twitch. In incomplete tetanus, muscles undergo quick cycles of contraction and brief relaxation. However, with higher stimulation frequency causing the relaxation phase to vanish, muscles contract continuously in complete tetanus. This continuous contraction is maintained by the high concentration of Ca2+ ions in the sarcoplasm, which allows sarcomeres to stay contracted until muscle fatigue sets in.